Synchronization signals exist to allow a wireless communication device operating in a cellular communication system, e.g. referred to as User Equipment (UE), to find the correct time and frequency relative a base station or access point, e.g. referred to as access network node or simply network node of a radio access network (RAN). Synchronisation signal are normally transmitted periodically, e.g. every 5 ms for 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system, but a forecast is that in coming systems, e.g. in 3GPP New Radio (NR) system it will likely be longer, e.g. 20 ms for initial access, but other systems and/or situations may have even longer periods, say up to 160 ms. The different systems, as indicated above, are considered to use different radio access technologies (RATs). The wideband code division multiplex access (WCDMA) is another example of a RAT for cellular communication. The NR synchronisation process is divided into two stages in which the first stage achieves time and a rough frequency control and the second stage provides a finer frequency control. Primary and secondary synchronization signals (PSS/SSS) are self-contained in that by successfully detecting one instance of PSS and one instance of SSS, the UE acquires information on physical cell identity and timing, after which it has sufficient knowledge to decode a physical broadcast channel (PBCH).
A key feature of the NR, which also is referred to as 5th Generation (5G) system, is to provide a lean carrier. That means that only a minimum of overhead in terms of broadcast messaging, or always-on messages will be transmitted. One consequence of that is that synchronisation signal deployment and detection will be slightly different compared with for example the LTE. In LTE, synchronisation signals are symmetrically positioned to the centre of the bandwidth, i.e., in a position relative to the network bandwidth, which implies that since it is possible to detect the network bandwidth, it is also possible to detect its centre frequency. This is schematically illustrated in FIG. 8. In NR, due to the lean design with few synchronisation signal instances per time and/or frequency, there may not be sufficient signal power to detect the synchronisation signals to detect the carrier bandwidth, or long averaging times would be required, another approach is desired. A consequence of that is that synchronisation signals may not necessarily be positioned symmetrically to the centre of the carrier, but may instead be positioned at predefined, absolute frequencies seemingly arbitrary to the network bandwidth. Furthermore, for network flexibility reasons, several synchronisation positions may exist within a network bandwidth, and may vary depending on configuration. This is schematically illustrated in FIG. 9.
It should be noted that the terms “network bandwidth”, “carrier bandwidth”, “network carrier bandwidth”, “system bandwidth”, etc. are interchangeably used in this disclosure and means the bandwidth in which the UE for the moment performs or intends to perform communication activities. Similarly, the terms “network frequency”, “carrier frequency”, “network carrier frequency”, “system frequency”, etc. are interchangeably used in this disclosure and means a frequency (reference within the bandwidth, e.g. at centre of the bandwidth) at which the UE for the moment performs or intends to perform communication activities.
Inter-RAT (IRAT) handover (HO) is the procedure of a UE changing RATs, from e.g., LTE to NR in active mode. Typically, this procedure also involves changing carrier frequencies and base station. Hence, one step of the IRAT HO procedure is to briefly change frequency to detect and synchronize towards the new RAT network. In legacy 4G systems such as the LTE, inter-frequency and IRAT mobility measurements and inter-frequency positioning, such as for example by observed time difference of arrival (OTDOA), are carried out in measurement gaps that are 6 ms long with periodicity of 40 or 80 ms depending on configuration. To simplify measurements, the UE in active mode may receive IRAT information in dedicated system information messages prior to the measurement, regarding what frequency to make measurements on and optionally candidate cells to search for.
The synchronisation signal in NR will not necessarily be on the system bandwidth (BW) on the carrier frequency. This is unlike what is the case in for example the LTE, where the sync signal always is centred around the carrier frequency. This might be a problem for multi-RAT UEs since during IRAT handover from systems like LTE or WCDMA to NR, the UE is unaware about where the synchronisation signal is allocated in the system BW. IRAT messages such as radio resource control (RRC) messages traditionally only constitute of carrier frequency information, and inherently RAT indication. If no synchronisation signal information is signalled, the UE need to search blindly for the synchronisation signal over the entire resources (time and frequency). The synchronisation time, i.e. time from IRAT measurements are triggered to when actual HO can be made, may be significantly delayed, which implies larger risk for dropped connection, lagging and hence worse user experience. It is therefore a desire to aid the UE in finding synchronisation signals more efficiently.